In order to attain a predetermined air-fuel ratio of air-fuel mixture to be burned in an internal combustion engine, the amount of air flowing into a cylinder (combustion chamber) of the internal combustion engine (hereinafter referred to as “cylinder intake-air quantity Mc”) must be accurately obtained. Usually, an air flow sensor is provided in an intake passage of the internal combustion engine, and the cylinder intake-air quantity Mc is estimated from an output value of the air flow rate sensor. However, when the internal combustion engine is in the state of transient operation; for example, when the opening of a throttle valve varies greatly with time, difficulty is encountered in accurately obtaining the cylinder intake-air quantity Mc from an output value of the intake-air flow sensor. Thus, in recent years, various attempts have been made to accurately estimate the cylinder intake-air quantity Mc by use of intake-system-related models that are represented by expressions obtained on the basis of hydrodynamics or the like (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 6-74076). FIG. 21 shows an intake-air quantity estimation apparatus that the applicants of the present application have been studying. The intake-air quantity estimation apparatus includes an electronic-control throttle model M10, a throttle model M20, an intake valve model M30, and an intake pipe model M40.
The cylinder intake-air quantity Mc becomes definite when the intake valve is closed (at intake-valve-closing time), and is proportional to pressure within the cylinder at this point of time. Pressure within the cylinder at intake-valve-closing time can be considered equal to pressure as observed upstream of the intake valve; i.e., equal to the air pressure Pm within an intake pipe (intake-pipe pressure). Thus, the intake-air quantity estimation apparatus shown in FIG. 21 is adapted to estimate the intake-pipe air pressure Pm at intake-valve-closing time by use of the models M10 to M40 and to estimate the cylinder intake-air quantity Mc from the estimated intake-pipe air pressure Pm.
More specifically, the electronic-control throttle model M10 is adapted to estimate the throttle-valve opening θt at intake-valve-closing time. The throttle model M20 is obtained on the basis of the law of conservation of energy, the law of conservation of momentum, the law of conservation of mass, and the equation of state and is adapted to estimate the flow rate mt of air passing through the throttle valve (throttle-passing air flow rate).
The intake valve model M30 is adapted to estimate a cylinder intake-air flow rate mc from the intake-pipe air pressure Pm, an intake-pipe air temperature Tm, an intake-air temperature Ta, and the like. In other words, as mentioned above, since the cylinder intake-air flow rate mc is considered proportional to the intake-pipe air pressure Pm, the intake valve model M30 obtains the cylinder intake-air flow rate mc in accordance with the following Eq. (1), which is obtained from an empirical rule (a rule of thumb).mc=(Ta/Tm)·(c·Pm−d)  (1)
In Eq. (1), c represents a coefficient of proportion, and d represents the amount of burned gas remaining in the cylinder (this value can be considered to be the amount of gas within the cylinder at exhaust-valve-closing time and is hereinafter referred to as merely “burned gas quantity d”). The intake valve model M30 stores tables (look-up tables or maps) that specify the relationships of the coefficient of proportion c and the burned gas quantity d, respectively, to the engine speed Ne, the intake-valve opening/closing timing VT, the intake-valve maximum lift Lmax, and the like. The intake valve model M30 obtains the coefficient of proportion c and the burned gas quantity d on the basis of the stored tables, the actual engine speed Ne, the actual intake-valve opening/closing timing VT, and the intake-valve maximum lift Lmax. At the time of computation, the intake valve model M30 estimates the cylinder intake-air flow rate mc by applying to Eq. (1) the last (latest) intake-pipe air pressure Pm and intake-pipe air temperature Tm at intake-valve-closing time, these last values having already been estimated by the intake pipe model M40, which will be described later.
The intake pipe model M40 is adapted to estimate the intake-pipe air pressure Pm at intake-valve-closing time by use of the throttle-passage air flow rate mt estimated by the throttle model M20 and the cylinder intake-air flow rate mc estimated by the intake valve model M30 and in accordance with expressions obtained on the basis of the law of conservation of mass and the law of conservation of energy, respectively. The intake-air quantity estimation apparatus is adapted to estimate the cylinder intake-air quantity Mc on the basis of the intake-pipe air pressure Pm at intake-valve-closing time, the intake-pipe air pressure Pm being estimated by the intake pipe model M40.
However, in the above-described intake valve model M30, the coefficient of proportion c and the burned gas quantity d are obtained by means of table searches involving a number of parameters, such as the engine speed Ne, the intake-valve opening/closing timing VT, and the intake-valve maximum lift Lmax; and the intake-air flow rate mc is estimated on the basis of the obtained coefficient of proportion c and burned gas quantity d. Thus, the intake valve model M30 involves the following problems: difficulty is encountered in determining the coefficient of proportion c and the burned gas quantity d for obtaining an accurate intake-air flow rate mc, with respect to all possible combinations of the parameters, the combinations totaling a huge number; and legitimacy check (adjustment) for c and d is very labor intensive.